Record the EUR rate of a non-EUR currency for one month (#922) as an append-only event. Body: {currency (ISO 4217, not EUR), month (YYYY-MM), rate (foreign units per 1 EUR, > 0), source (required — where the rate is from, e.g. the official monthly table)}.

A recorded rate is immutable: a second record for the same month is refused (409 rate_exists) unless it says supersedes: true with a reason — then it is recorded with the rate it replaces, and a billing.fx_rate.superseded audit event is written. The export flags every row converted at a superseded rate (eur_rate_superseded, superseded_rates): an export taken before may differ.